Asylum seekers are questioning the Federal Government's threat to send them offshore despite them arriving on Australian soil before Parliament passed new legislation.

The Hazara asylum seekers from Afghanistan were rescued by a container ship when their boat sank.

The cargo ship had wanted to take them to Singapore, where the ship was headed, but the captain said he decided take the group to Christmas Island after threats of self-harm.

In an open letter written from inside Immigration detention, the asylum seekers say they they were dropped off at Christmas Island on August 14, three days before the Parliament passed the offshore processing laws.

They argue they are genuine refugees who have suffered terrible persecution and they fear if they are sent offshore their mental health will deteriorate.

The Government has said it expects to start moving the first asylum seekers from Christmas Island this month.

The Government says more than 1,500 people who have arrived since the August 13 deadline risk being sent offshore.